Method of detecting and classifying scratches, particles and pits on thin film disks or wafers

ABSTRACT

Scratches, pits and particles which are smaller or larger than the beam size may be measured and identified by a dual beam technique. This invention uses a pair of orthogonally oriented laser beams, one in the radial and one in the circumferential direction. The scattered light from radial and circumferential beams allows the detection and classification of particles, pits and scratches.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/126,154 filed on Apr. 19, 2002, which is acontinuation-in-part of U.S. patent application Ser. No. 10/029,957filed on Dec. 21, 2001, which is a continuation-in-part of U.S. patentapplication Ser. No. 09/861,280 filed on May 18, 2001 now U.S. Pat. No.6,757,056, which is a continuation of U.S. patent application Ser. No.09/818,199 filed on Mar. 26, 2001, which are all incorporated byreference herein in their entirety.

This application is also related to U.S. patent application Ser. No.09/718,054 filed on Nov. 20, 2000, which is a continuation-in-part ofU.S. patent application Ser. No. 09/414,388 filed on Oct. 7, 1999, whichis a continuation-in-part of U.S. patent application Ser. No. 09/347,622filed on Jul. 2, 1999, which is a continuation-in-part of U.S. Pat. No.6,031,615, which claims priority from provisional application number60/059,740 filed on Sep. 22 1997, which are all incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed toward measuring thin films anddefects on silicon wafers, magnetic thin film disks and transparent andcoated glass substrates and more particularly toward measuring thin filmthickness, and wear, surface roughness, scratches, particles, stains,pits, mounds, surface topography, step heights, and inclusions using alaser directed toward a thin film disk at many angles includingnon-Brewster's angles of an absorbing layer of the thin film.

2. Description of Background Art

Coated thin film disks are used in a variety of industries including thesemiconductor and the magnetic hard disk industry. A computer hard disk(magnetic storage device) is a non-volatile memory device that can storelarge amounts of data. One problem that the manufacturers of hard disksexperience is how to maximize the operating life of a hard disk. When ahard disk fails the data stored therein may be difficult, expensive, orimpossible to retrieve. Failure of a hard disk may be caused by defectson the surface of the thin film disk. It is important to be able todetect and classify these defects in order to prevent disk drive failureand to control the manufacturing process.

A schematic of a thin film disk used in magnetic storage devices isshown in FIG. 1. It includes a magnetic thin film (layer) 106 which isdeposited upon a substrate 108 (typically a NiP plated Al—Mg alloy orglass). The magnetic thin film 106 can be protected by a thin film ofcarbon 104 (carbon layer), for example, whose thickness is typically 20to 200 Angstroms (Å). The carbon layer 104 is typically coated with athin layer (10 to 30 Angstroms) of a fluorocarbon lubricant 102(lubricant layer). The lubricant layer 102 serves to increase thedurability of the underlying carbon layer 104 particularly when themagnetic read/write head contacts the disk, for example when the diskdrive is turned off. The hard disk drive industry has been dramaticallyimproving storage capacity by flying the thin film head closer to thesurface of the thin film disk. As a result even very small defects cancause a hard drive to fail. These defects may be topographic such asscratches, pits, mounds, or particles or they may be non-topographicsuch as stains or inclusions. It is useful to measure all these types ofdefects to control the disk manufacturing process and improve disk drivemanufacturing yield.

A schematic of a semiconductor wafer is shown in FIG. 2. The structureof a semiconductor wafer can be very complex and FIG. 2 shows only atypical structure of a wafer that is undergoing the copper dualdamascene process. In FIG. 2, 201 is the copper layer 202 is the secondplasma enhanced chemical vapor deposited (PECVD) oxide layer, 203 is thefirst PECVD oxide layer and 204 is the silicon substrate. The copperlayer 201 is polished down using a chemical mechanical polishing (CMP)process until only the via holes and copper lines remain. The problem isthat the CMP process can leave residual copper, nitride, or CMP slurryon the surface of the wafer. In addition, stains, particles, scratches,and micro-waviness may be present on the polished wafer. It is useful todetect and measure such defects to control the process of making thewafer. Fewer defects will also mean greater wafer yields at the end ofthe process. The problem in the hard disk, semiconductor and photonicsindustries is to inspect these magnetic disks and wafers for defectssuch as particles, scratches, pits, mounds, stains, topographicirregularities and inclusions. Conventional techniques to solve theseproblems are discussed in U.S. Pat. Nos. 4,674,875, 5,694,214,5,748,305, and 6,157,444 that are all incorporated by reference hereinin their entirety. These patents describe techniques to measure defectsusing essentially sophisticated scatterometers and reflectometers. Noneof these systems enables the simultaneous measurement of topographic andnon-topographic defects. The present invention enables this measurementthrough the use of a combined reflectometer, scatterometer,ellipsometer, profilometer and Kerr effect microscope.

What is needed is a system and method for examining thin film disks,silicon wafers and transparent wafers that: (1) measures topographic andnon-topographic defects; (2) measures the optical profile on thesesubstrates; (3) enables the measurements to be performed simultaneously;(4) measures the thickness of thin films; (5) enables measurement onpatterned or unpatterned silicon or photonic wafers; (6) is performed insitu or in line; (7) measures only a single side of a transparentsubstrate or (8) is configurable to have multiple selectable beam widthsto test varying spot sizes of the object being examined.

As the technology for the semiconductor and the disk drive industriescontinues to advance there is a need to detect and classify ever smallerdefects. When a defect is smaller in size than the dimension of ameasurement beam, it is difficult to determine the nature of the defect.What is needed is a method for identifying and classifying defectsregardless of the relative size of the measurement beam.

SUMMARY OF THE INVENTION

A method is provided for categorizing defects, such as scratches,particles, and pits, on the surface of an object. Two or more lightbeams with different planes of incidence, such as orthogonally orientedbeams, are directed at the surface of the object. The scattered lightintensities produced when the beam strikes a defect are measured by adetector. The scattered light intensities of the beams are compared todetermine the aspect ratio of the defect. The method can categorizedefects that are smaller than the size of the beam spot on the objectsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a thin film that can be measured using anembodiment of the present invention.

FIG. 2 is an illustration of a semiconductor wafer that can be measuredwith one embodiment of the present invention.

FIG. 3 is one half of optical layout of combined ellipsometer andoptical profiler (side view).

FIG. 4 is a top view of an optical profilometer that measures height orslope according to one embodiment of the present invention.

FIG. 5 is a top view of an optical profilometer having a single laserthat measures height or slope according to another embodiment of thepresent invention.

FIG. 6 is a side view of optical profilometer showing laser one and PSD1 according to one embodiment of the present invention.

FIG. 7 illustrates the height sensitivity multiplier as a function ofangle of incidence (theta) according to one embodiment of the presentinvention.

FIG. 8 is an illustration of a miniature optical surface analyzeraccording to one embodiment of the present invention.

FIG. 9 is an illustration of a miniature optical surface analyzeraccording to another embodiment of the present invention.

FIG. 10 is an illustration of a miniature optical surface analyzeraccording to another embodiment of the present invention from a top viewperspective.

FIG. 11 is an illustration of a miniature optical surface analyzer ofFIG. 10 from the perspective from the A direction.

FIG. 12 is an illustration of a miniature optical surface analyzeraccording to another embodiment of the present invention from a top viewperspective.

FIG. 13 is an illustration of a final test spindle with dual miniatureoptical heads and stepper motor according to one embodiment of thepresent invention.

FIG. 14 is an illustration of a spatial filter for blocking bottomsurface reflection from a glass substrate according to one embodiment ofthe present invention.

FIG. 15 is an illustration of one half of an optical layout of combinedellipsometer and optical profiler from a side view perspective accordingto one embodiment of the present invention.

FIG. 16 is an illustration from a top view perspective of a combinedellipsometer and optical profilometer according to one embodiment of thepresent invention.

FIG. 17 is an illustration of a system for measuring the phase shift ofan elliptically polarized beam by use of a beam splitter that splits thebeam into non-orthogonally polarized components according to oneembodiment of the present invention.

FIG. 18 is a graph illustrating the reflectivity versus angle ofincidence for copper and glass with polarization as a parameteraccording to one embodiment of the present invention.

FIG. 19 is an illustration of one half of a material independent opticalprofilometer from a side view perspective according to one embodiment ofthe present invention.

FIG. 20 is an illustration of another half of a material independentoptical profilometer from a side view perspective according to oneembodiment of the present invention.

FIG. 21 is an illustration of one half of a material independent opticalprofilometer that uses incident light that is circularly polarizedaccording to one embodiment of the present invention.

FIG. 22 is an illustration having a top view perspective of an opticalprofilometer that is completely material independent according to oneembodiment of the present invention.

FIG. 23 is an illustration having a top view perspective of a materialindependent optical profilometer using a single laser as its opticalsource according to one embodiment of the present invention.

FIG. 24 is an illustration having a top view perspective of an opticalprofilometer that is completely material independent according toanother embodiment of the present invention.

FIG. 25 is an illustration having a top view perspective of an opticalprofilometer that is completely material independent according toanother embodiment of the present invention.

FIG. 26 is an illustration of a pair of material independent opticalprofilometers that are arranged at 90° to cancel pattern effects onpatterned semiconductor wafers according to one embodiment of thepresent invention.

FIG. 27 is an illustration of an optical profilometer that cancels slopeand measures height using circularly polarized light incident upon asample according to one embodiment of the present invention.

FIG. 28 is an illustration of an optical profilometer that cancels slopeand measures height using a single detector and using S polarized lightincident upon a sample according to one embodiment of the presentinvention.

FIG. 29 is an illustration of an optical profilometer that cancels slopeand measures height using a single detector an using 45° linearlypolarized light incident upon a sample according to one embodiment ofthe present invention.

FIG. 30 is an illustration of an optical profilometer that measures onlyslope and cancels height and material effects according to oneembodiment of the present invention.

FIG. 31 is an illustration of an optical profilometer that cancels slopeand measures height using a single detector and using S polarized lightincident upon a sample according to another embodiment of the presentinvention.

FIG. 32 is an illustration having a side view perspective of anoptically scanned version of a material independent optical profilometershown in FIGS. 19, 20, 21 and 22 according to one embodiment of thepresent invention.

FIG. 33 is an illustration having a top view perspective of an opticallyscanned material independent optical profilometer according to anotherembodiment of the present invention.

FIG. 34 is an illustration depicting the beam profiles after one and tworeflections from the surface under measurement according to oneembodiment of the present invention.

FIG. 35 is an illustration of mode modulation circuitry according to oneembodiment of the present invention.

FIG. 36 is an illustration of a material independent opticalprofilometer having one detector according to one embodiment of thepresent invention.

FIG. 37 is an illustration from a top view perspective of a materialindependent optical profilometer having one detector according to oneembodiment of the present invention.

FIG. 38 is an illustration of an optical profilometer, ellipsometer,reflectometer and scatterometer which uses a telescope to produce userselectable multiple spots sizes on a substrate.

FIG. 39 is an illustration of a method of detecting the thickness of adisk or wafer.

FIG. 40 is an illustration of a thickness detector integrated with anoptical surface analyzer.

FIG. 41 is an illustration of the scattered light detection of aparticle, pit, and scratch using two orthogonal beams.

FIG. 42 is an illustration of the scattered light signals for aparticle, pit and a scratch from both the radial and circumferentialbeams.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention is now described withreference to the figures where like reference numbers indicate identicalor functionally similar elements. Also in the figures, the left mostdigit(s) of each reference number correspond(s) to the figure in whichthe reference number is first used.

FIG. 3 is an illustration of an apparatus for measuring properties ofthe thin film according to an embodiment of the present invention. Theapparatus uses a focused laser light signal whose angle of propagation θ(as shown in FIG. 3) can be between zero degrees from normal and ninetydegrees from normal.

One embodiment of the apparatus 300 includes a conventional laser diode301, e.g., RLD65MZT1 or RLD-78MD available from Rolm Corporation, Kyoto,Japan, which has been collimated by Hoetron Corp., Sunnyvale, Calif.,e.g., a conventional linear polarizer 302, e.g., made of Polarcor thatis commercially available from Newport Corp., Irvine, Calif., aconventional zero order half wave plate 303 that is commerciallyavailable from CVI Laser, Livermore Calif., a conventional focusing lens304 that is commercially available from Newport Corporation, Irvine,Calif., conventional mirrors 305 and 306 available from Newport Corp.Irving, Calif. A collimating lens 309 available from Newport Corp., azero order quarter wave plate 310 available from CVI Laser Corp., aconventional polarizing beam splitter 312 rotated at 45° to the plane ofincidence available from CVI Laser Corp., a pair of conventionalquadrant detectors 311 and 313 available from Hamamatsu Corp., HamamatsuCity, Japan, a conventional avalanche photodiode 314 available fromAdvanced Photonix, Inc., Camarillo, Calif. and a conventional motor 315available from Maxon Precision Motors, Burlingame, Calif. for rotatingthe half wave plate 303. The avalanche photodiode 314 may be replacedwith a conventional photo multiplier tube (PMT) available from HamamatsuCorp., Hamamatsu City, Japan.

It will be apparent to persons skilled in the art that the apparatus 300is an embodiment of the present invention and that alternate designs canbe used without departing from the present invention. The operation ofthe apparatus 300 is now described in greater detail.

A laser diode 301 emits an electromagnetic signal toward the thin filmdisk, silicon wafer, photonics wafer or glass substrate. In anembodiment the electromagnetic signal is a light signal having awavelength of 780 or 655 nanometers (nm) although a wide variety ofwavelengths can be used. The angle of propagation of the light signalcan be any angle θ between zero and ninety degrees.

Laser diodes are well known to have an internal photodiode to monitorthe laser output power. An embodiment of a feedback control circuit tocontrol the optical intensity is to use such a photodiode, which isinternal to the laser diode. This photodiode which is internal to thelaser diode feeds back a control signal to negative feedback circuitryand by doing so keeps the intensity of the laser at a constant value.The photodiode that is used to control the laser intensity may beexternal to the laser. When an external photodiode is used an externalnon-polarizing beam splitter is placed after the laser. This externalnon-polarizing beam splitter directs a sample of the laser onto theexternal photodiode. The signal from the external photodiode is used tofeedback a control signal to negative feedback circuitry and therebycontrols the laser intensity. Another means of keeping an approximateconstant output power of the laser is to control the current of thelaser diode, that is, run the diode laser in a constant current mode.The laser diode will exhibit a very slow decrease in output power over aperiod of months. As long as the scan time is less than 5 or 10 minutesthen the optical power output of the laser will remain constant duringthe scan. The advantage of this technique is its simplicity. Long-termdrifts of the laser output power may be calibrated out of the system byfirst measuring a standard reflector and using this to normalize themeasured signals. The value of the signal is first measured over thestandard (known) reflector and then the disk or wafer is measured. Ifthere has been any drift of the standard reflector measurement then allthe data is corrected for this amount of drift. As a result long-termdrifts may be compensated even when operating in a constant currentmode. The emitted light passes through the linear polarizer 302. Thelinear polarizer 302 improves the linear polarization of the laser lightsignal.

There are several ways to reduce the optical noise of lasers. One ofthese is to use a multi-mode laser diode (such as the Rohm laser diodementioned above) that runs in 6 to 8 longitudinal modes simultaneously.This prevents the laser from mode hopping and reduces intensity noise.Another way to reduce noise is to start with a single mode laser and tomodulate the laser current at a frequency from 30 to 1000 MHz. The lasercurrent includes a DC component of 20 to 100 ma plus a smaller ACcomponent at the above specified frequency. The AC component of thecurrent forces the single mode laser to run in several modes and thisprevents mode hopping and reduces laser noise. This technology is knownas noise reduction through mode modulation. A third way to reduce noiseis to use a thermoelectric cooler (TEC) to keep the laser temperatureconstant. The TEC technology will reduce mode hopping but will notprevent it. The TEC technology will also increase the diode laserlifetime.

The mode modulation technology is useful in instruments like the OpticalSurface Analyzer discussed herein. This is because the laser noise andintensity stability limits the sensitivity of the instrument. The bestway to eliminate mode hopping is to use mode modulation. FIG. 35 shows aschematic of the mode modulation technology. The 30 to 1000 MHzmodulation comes from the AC source and the DC source provides the 20 to100 ma DC current needed to run the laser. The blocking capacitorprevents the DC current from passing into the AC source. When thistechnology is combined with the Optical Surface Analyzer describedherein the sensitivity of the instrument can be greatly improved.Further improvements may be achieved by combining TEC with modemodulation technology.

The linearly polarized light passes through a mechanically rotatablezero order half-wave plate 303. The half wave plate 303 is attached to aminiature motor 315 which allows the polarization to be dynamicallyrotated between P polarized (parallel to the plane of incidence), Spolarized (perpendicular to the plane of incidence) and 45° polarized(between P and S) light. The polarized light passes through a focusinglens 304 and is directed onto a thin film magnetic disk, silicon waferor transparent substrate 306 by a turning mirror 305. The reflectedsignal is directed to the detection optics by another turning mirror 308and recollimated by another lens 309. An avalanche photodiode,conventional PIN photodiode or photo multiplier tube 314, for example,detects the scattered component of the signal. The recollimated beampasses through a zero order quarter wave plate 310 that is used toadjust the polarization of the beam so that equal amounts of energy aredirected into the quadrant photodetectors 313 and 311. After passingthrough the quarter wave plate 310 the beam is split by a polarizationbeam splitter 312 that is rotated by 45° to the plane of incidence. Inanother embodiment the polarizing beam splitter may be a Wollaston prismor a Glan Thompson or a Rochon prism beam splitter. The split beams aredirected onto two quadrant detectors 311 and 313. The quadrant detectorsare used to compute the phase shift between the split beams, thereflectivity, the optical profiles in the radial and circumferentialdirections, and the Kerr rotation (if the film on the substrate 306 ismagnetic). The outputs from the quadrant detectors are digitized by aconventional analog to digital converter and directed to the memory of aconventional personal computer. The signals are then analyzed by thepersonal computer to detect defects, measure topography, and measurestains. The entire optical apparatus 300 is placed upon a stage thatmoves the apparatus in the radial direction while a motor 307 rotatesthe sample 306. In this manner the entire surface of the sample 306 maybe scanned for defects.

An alternative embodiment for scanning the entire substrate 306 is toplace the optical head or the substrate 306 on a x-y scan stage. Thesubstrate 306 or the optical apparatus 300 is scanned in the x and ydirections and in this manner the entire sample may be scanned fordefects or topography.

The spindle or motor which rotates the disk at a high rate of speedincludes an encoder which produces 1024 pulses as it rotates through 360degrees, for example. This encoder is used to determine thecircumferential positions around the disk. The present inventionpreferably utilizes a very high-resolution determination of the positionaround the circumference of the disk. This is accomplished by using aphase locked loop to multiply the encoder signal by a selectable factorof up to 64 times. The phase locked loop, which multiplies the 1024encoder pulses, has the ability to track any velocity jitter in theencoder. This feature allows averaging of repeated revolutions to bedone with no loss of lateral resolution. That is, subsequent revolutionslie in phase with one another and when averaged, the resulting image isnot smeared by any jitter effect. Averaging is done to improvesignal-to-noise ratio.

FIG. 4 shows the top view design of an optical profilometer, whichmeasures height only and measures height directly. It can also measurethe slope of the surface independent of height. This differs fromprevious optical profilometers that measure both slope and height at thesame time. With such systems the height is obtained from the slope databy integrating the slope information. However, if the slope informationis contaminated with height information then the integration will notgive the correct surface profile. A goal is to obtain data that includesonly height information and not a combination of both slope and height.The design illustrated and described with reference to FIGS. 4-7accomplishes this by using two lasers and two position sensitivedetectors (PSD) oriented at right angles to one another.

The position sensitive detectors (PSD) are quadrant detectors that areoriented as shown in FIG. 4. The PSD's measure the displacement of thebeam in the radial and circumferential directions by subtracting theappropriate PSD quadrants. As the laser beam moves along the surface ofthe object to be measured, the roughness and waviness of the surfacecause the laser beam to “wiggle” on the quadrant detector in response tothe slope of the surface. The quadrant detector measures this bysubtracting the sum of one pair of quadrants from the sum of anotherpair. For example, referring to FIG. 6, the slope of the surface in thecircumferential direction is given by [(A1+B1)−(C1+D1)]/[A1+B1+C1+D1]where the sum of the denominator is used to normalize for reflectivitydifferences. At the same time, if the average distance of the surfacefrom the detector changes, then the average position of the beam on thequadrant detector will change. The resulting difference signal in theabove equation will register a slope change when in fact a difference insurface height is occurring. The problem is to be able to separate slopechanges from height changes. This can be accomplished by considering theslope in the radial direction, which is obtained by referring to FIG. 6and is given by [(A1+D1)−(B1+C1)]/[A1+B1+C1+D1]. The equation for theradial slope measures the “wiggle” of the beam in the radial direction.In the case of the radial slope, if the average distance of the surfacefrom the detector changes then the beam simply moves along the lineseparating A1+D1 from B1+C1. As a result the radial slope signal doesnot change when the surface height changes and the equation for theradial slope records only slope and not height changes.

When the orientation of the laser beam is rotated by 90 degrees (as withlaser 2 and PSD 2 in FIG. 4) the behavior of the radial andcircumferential slope will reverse. In the case of laser 2 and PSD 2 thecircumferential slope equation will record only slope changes and notheight changes. On the other hand, for laser 2, the radial slopeequation will record both slope and height changes. Since the outputbeam of both lasers 1 and 2 is positioned at the same location on thesurface (as shown in FIG. 4) then it is possible to subtract the radialslope equation from laser 1 and PSD 1 from the radial slope equationfrom laser 2 and PSD 2. The resulting subtraction will include onlyheight information and no slope information. It is also possible toobtain the same information by subtracting the circumferential slopeequation from laser 1 and PSD 1 from the circumferential slope equationfrom laser 2 and PSD 2. The radial slope (with no height information)can be obtained by choosing the radial slope equation from laser 1 andPSD 1. The circumferential slope (with no height information) can beobtained by choosing the circumferential slope equation from laser 2 andPSD 2. In this manner it is possible to independently measure surfaceheight variation and slope variation.

In another embodiment of this optical profilometer, as shown in FIG. 5,a single laser is used and a 50/50 mirror 504 oriented at a compoundangle directs a second beam onto the surface to a position labeled 502on FIG. 5. The beam that passes through the 50/50 mirror 504 is directedonto the surface to a position labeled 501 on FIG. 5. The entire surfaceof the object to be measured is scanned with both of the beams resultingin at least two images of the surface. The resulting images are storedand digitally shifted so that the resulting images have the object to beprofiled at the same x, y location. The resulting shifted images maythen be subtracted to give the height profile in the manner describedabove. The advantage of this embodiment is that it uses only a singlelaser and fewer optical components and the beam shape of the two beamsis identical.

Laser one and PSD 1 nominally measure the signal in the radial, Sr, andthe signal in the circumferential, Sc, directions. However, the natureof the PSD results in Sc from laser one and PSD 1 being contaminatedwith height information, in addition to slope information. Sr from laser1 and PSD 1 include only slope information. Laser two and PSD 2 alsonominally measure the slope in the radial and circumferentialdirections. However, Sr from laser 2 and PSD 2 measures both slope andheight at the same positions as Sr from laser 1 and PSD 1. As a resultthe true height variation can be obtained by subtracting Sr from laser 2and PSD 2 from Sr from laser 1 and PSD 1. That is, the slope informationis removed when subtracting Sr from PSD 2 from Sr from PSD 1, leavingonly the height information.

A similar result can be obtained from subtracting Sc from PSD 2 thatonly includes slope information. As a result, subtracting Sc from PSD 2from Sc from PSD 1 gives data including only height information. Theresult is a direct measurement of height. The advantages of thistechnique are that it gives a direct measurement of height and it can bedone in a non-contact manner at high speed. This technique can alsomeasure step heights with 90-degree step angles. Conventional systems,which use slope measurements, cannot measure 90-degree step heights.

FIG. 6 shows the side view design of the optical profilometer. Thisfigure only shows laser 1 and PSD 1 in an effort to easily show the sideview design. In FIG. 6 the optical profilometer is positioned above athin film disk or wafer and is translated in the radial direction whilethe disk or semiconductor wafer is rotated.

The angle of incidence (θ) shown in FIG. 6 can be chosen for theparticular application. Any angle of incidence can be chosen exceptnormal incidence, where the PSD's would have no height sensitivity. Foran application that involves transparent substrates one could chooseangles greater than 45 degrees in order to increase the reflectionsignal from the surface. As the angle of incidence increases, the heightsensitivity also increases by the factor 2 sin θ. A plot of this factoris shown in FIG. 7. This suggests that an angle of incidence greaterthan or equal to approximately 60 degrees would be optimal, although notnecessary. At angles greater than 60 degrees the sensitivity willincrease and the signal from a transparent surface will increase. Thisembodiment requires that the focused spot sizes of the two lasers besubstantially identical and that the laser spots overlap as closely aspossible.

A problem in the magnetic recording industry is to inspect thin filmdisks for defects at the final test step of the manufacturer of disks.The manufacturers of thin film disks require that both sides of the thinfilm disk be inspected simultaneously. The problem is that the clearancebetween the disk and the chuck (which holds the disk) is only 1″ or less(see FIG. 13, 1304). This requires that the optics be miniaturized inorder to fit in the small space between the disk and the chuck (see FIG.13). A solution to this problem can be obtained by using the opticaldesigns in FIGS. 8, 9, 10, and 11. These designs have several keyimprovements, which allow the design to be miniaturized withoutcompromising the performance of the device. First of all the design usesthe internal feedback photodiode, which is included within the laserdiode 801, to achieve stabilization of the DC level of the opticalsignal. Secondly, the angle of incidence, θ, is adjusted to reduce theheight of the instrument so that it will fit within the 1″ spacerequirement. Thirdly, the surface topography measurement capabilityfeature of the instrument is incorporated within the phase/speculardetectors 808 and 809 shown in FIGS. 8 and 9. The position sensitivedetectors 808 and 809 (quadrant detectors) serve as both phasedetectors, specular detectors, and topography measurement detectors.Fourthly, the size may be decreased by using a polarizing beam splitter901 as shown in FIG. 9 instead of a Wollaston prism 807 as shown in FIG.8. The polarizing beam splitter 807 or Wollaston prism 901 is rotated at45° with respect to the plane of incidence.

Another embodiment of this invention can use a beam splitter that splitsthe beam into non-orthogonal components, which will be discussed in asubsequent section. Using two spherical mirrors 1004 and 1006 to directthe beam onto the disk as shown in FIG. 10 will diminish the size in thelateral dimension. The mirrors 1004 and 1006 are adjusted at a compoundangle as shown in FIG. 10. This is also shown in FIG. 11 which is a viewof FIG. 10 along the “A” direction, where the mirrors that are at acompound angle are 1102 and 1104. These mirrors direct the beam 1103onto the disk or wafer 1101. In addition to directing the beam onto thedisk the spherical mirrors also focus the beam to a small spot. In analternative embodiment flat mirrors 1202 and 1203 are used incombination with focusing lenses 1201 as shown in FIG. 12. Also shown inFIG. 12 is a silicon photodetector or avalanche photodiode or photomultiplier tube 1204, which is positioned above the point where the beamstrikes the disk. This element enables the detection of submicronparticles. The avalanche photodiode 1204 is available from AdvancedPhotonix, Inc., Camarillo, Calif.

Referring to FIG. 8, the laser beam from the diode laser 801 passesthrough a linear polarizer 802, and a focusing lens 803 and then strikesa disk or wafer 804. Upon reflecting from the surface the beam passesthrough a recollimating lens 805, a quarter wave plate 806, and througha polarizing beam splitter such as Wollaston prism 807 which is rotatedat 45° to the plane of incidence and onto two quadrant detectors 808 and809. The specular signal is obtained by summing the signals fromposition sensitive detector 1 809 with the sum of position sensitivedetector 2, 808 times a constant κ:

 Specular signal=(A 1 +B 1 +C 1 +D 1)+κ*(A 2 +B 2 +C 2 +D 2)

The cosine of the phase shift between the two split beams (Cos(ps)) canbe obtained by subtracting the sum of the elements of detector 1809 fromthose of detector 2, 808 times a constant κ:Cos(ps)=(A 1 +B 1 +C 1 +D 1)−K*(A 2 +B 2 +C 2 +D 2) where K is aconstant.Referring to FIG. 8 detector 1, 809, the slope in the circumferentialdirection is given by:Slope in circumferential direction=[(B 1 +C 1)−(A 1 +D 1)]/(A 1 +B 1 +C1 +D 1)The slope in the radial direction is given by:Slope in the radial direction=[(A 1 +B 1)−(C 1 +D 1)]/(A 1 +B 1 +C 1 +D1)

The topography in the circumferential or radial direction is obtained byintegrating the slope in the circumferential or radial direction,respectively. The slope signals can also be obtained from detector 2,808 with the same equations as shown above except for substituting 2 for1.

Using the designs in FIGS. 8, 9, 10 and 12 will allow the measurement ofsub-micron scratches, particles, stains, pits, mounds, handling damage,wear of the carbon layer, outside diameter damage and contamination.This design can also measure the longitudinal Kerr effect by ameasurement of the Kerr rotation angle. The advantages of this designare its small size which is made possible by detectors which combine themeasurement of phase shift, specular reflectivity, radial andcircumferential slope, and scattered light.

The miniature optical design may be mounted on the top and bottom of athin film disk 1302 as shown in FIG. 13 and the resulting combination istranslated over the surface of the disk with a stepper or DC servomotordriven stage 1308. A spindle motor 1306 rotates the disk while theoptics 1301 is translated in the radial direction so that 100% of thesurface of the disk may be measured for defects. The entire apparatus ismounted on a baseplate 1307. The electronics package is located abovethe stepper motor 1303. The disk is placed upon a vacuum chuck 1305 thatis rotated at a high rate of speed.

A problem in the inspection of transparent glass substrates 1406 andother transparent objects is to separate the signal from the top and thebottom surface. This can be accomplished by the use of a spatial filter1404 that blocks the signal from the bottom surface 1405 and does notaffect the top surface reflection 1403. FIG. 14 shows this in theoptical design of the Optical Surface Analyzer (OSA). The incomingoptical beam is 1401.

The spatial filter 1404 is in the shape of a small wedge that isattached to the bottom surface of the integrating sphere 1402. Thelocation of the spatial filter is adjusted to just block the bottomsurface reflection 1405 and not to interfere with the top surfacereflection 1403. This invention allows one to separate information fromthe top and bottom surface of a transparent glass disk or wafer 1406.This invention also works with any transparent medium such as lithiumniobate, fused silica, photoresist, and other transparent oxides.

An alternative design does not require the spatial filter to be attachedto the bottom of the integrating sphere. For example, the integratingsphere may be omitted and the spatial filter may be attached to anyother point on the optical body. The crucial point is that the spatialfilter must be located near enough to the transparent substrate so thatthe reflections from the top and bottom surface are separated in thelateral plane. In this manner it is possible to intercept the bottomsurface reflection with the spatial filter and leave the top surfacereflection unaffected.

A problem in the measurement of semiconductor wafers is the detection ofdefects caused by the CMP (Chemical Mechanical Polishing) process. Thesedefects can be residual copper, nitride, slurry, particles, scratchesand stains. The measurement is complicated by the fact that thesemiconductor wafers have a very complex pattern on their surface. Theobject is to separate the defects from the complex pattern ofsemiconductor devices on the surface of the semiconductor wafer. Thiscan be accomplished by the design shown in FIG. 15. The device includesa means for measuring the phase shift between the P and S polarizationcomponents of the incident beam and a means to measure the topography ofthe surface. The device includes a laser 1501 and a polarizer 1502. Thelaser is directed onto a focusing lens 1503 and onto a mirror 1504 thatdirects the beam onto a wafer or disk 1505 that may be rotated by amotor 1506. The reflected beam is directed by another mirror 1507 onto acollimating lens 1508 and through a quarter wave plate 1509. The signalpassing through the quarter wave plate is directed onto a polarizingbeam splitter 1511 that is oriented at 45° to the plane of incidence.The split beams are measured with two photodetectors 1510 and 1512. Thephase shift of the incident beam is proportional to the difference inthe amplitudes of photodetectors 1510 and 1512.

When the phase shift between the split beams is measured it is foundthat the orientation of the semiconductor pattern lines will have asubstantial effect on the measured phase shift. What is desired is toremove the semiconductor pattern and enhance the defects. A means toaccomplish this is to image the wafer with two orthogonal beams as shownin FIG. 16. An optical path shown in FIG. 15 generates each of the beamsshown in FIG. 16. Laser one 1601 and detector one 1602 in FIG. 16generate a phase shift image of the surface that has one particularamplitude due to the orientation of the semiconductor pattern lines.Laser two 1603 and detector two 1604 have a particular amplitude patternthat is identical in lateral shape but opposite in amplitude to thatgenerated by laser one 1601 and detector one 1602. This is because theorientation of the optical beams of lasers one and two are orthogonalwith respect to the orientation of the pattern lines. As a result, whatis generated are two phase shift images of the surface of thesemiconductor that have opposite amplitude phase shift signals from thesemiconductor pattern lines. If these two images are added together thenthe semiconductor pattern will be greatly attenuated. Defects, on theother hand, do not change phase shift in the two orthogonal beams and asa result when the two orthogonal images are added the defects increasein amplitude and the semiconductor pattern diminishes in amplitude.Defects do not have opposite phase shift amplitudes since most defectsare isotropic in nature and do not have the strong anisotropy associatedwith semiconductor pattern lines. This technique effectively enhancesthe defect signals and diminishes the semiconductor pattern signal. Thefocused beams 1607 cross at point 1606. The entire device is includedwithin housing 1605.

This invention has the additional advantage that it can simultaneouslymeasure the topography of the surface as has been described in U.S.patent application Ser. No. 09/718,054 which is incorporated byreference herein in its entirety. In the preferred embodiment the angleof incidence (θ) shown in FIG. 15 is at approximately 60°. Larger orsmaller angles of incidence may be used depending upon the application.For example, a larger angle of incidence may be used if a transparentsubstrate is to be examined. This would be advantageous since atransparent substrate will give a larger signal from the top surfacewith a greater angle of incidence. Simultaneous measurement at two ormore angles of incidence may be accomplished by making the angle ofincidence of laser 1601 at a first angle θ₁ and that of laser 1603 at asecond angle θ₂, This will involve changing the angle of the turningmirrors 1504 and 1507 for both lasers 1601 and 1603. The angle ofincidence θ₁ or θ₂ may be between zero and 90 degrees. This particularembodiment allows two angles of incidence to be simultaneously scanned.The simultaneous scanning of additional angles of incidence may beobtained by adding additional lasers in FIG. 16 at angles between theorthogonal pair of lasers 1601 and 1603. Each laser added between 1601and 1603 may be adjusted to be incident on the surface at any angle ofincidence between 0 and 90 degrees.

Simultaneous measurement at two or more wavelengths may be accomplishedby making each laser 1601 and 1603 a different wavelength. In thismanner phase shift and reflectivity information may be simultaneouslycollected at two wavelengths. Additional wavelengths may be added bypositioning additional lasers and detectors between the orthogonallyoriented lasers 1601 and 1603. Each laser added between 1601 and 1603will have a different wavelength so that any number of wavelengths maybe simultaneously incident upon the substrate or disk 1505.

The advantage of multiple wavelengths or angles of incidence is thateach angle or wavelength gives different information on the propertiesof the substrate or disk 1505. For example, shorter wavelengths willallow the detection of smaller particles and thinner films.

FIG. 17 illustrates the measurement of the phase shift of anelliptically polarized beam by the use of a beam splitter that splitsthe beam into non-orthogonally polarized components. The incomingelliptically polarized beam is labeled 1701, this beam is directed intoa quarter wave plate 1702 and subsequently into a beam splitter 1703which splits the beam into nonorthogonally polarized components.Internal to 1703 is a polarizing beam splitter such as a Wollaston prism1704 or a polarizing cube beam splitter and a polarization rotationdevice 1705 such as a half wave plate or an optically active quartzpolarization rotator. The two beams leaving the beam splitter 1703 arepolarized in the same direction as indicated by 1706 and 1707. Ingeneral the two beams leaving the beam splitter 1703 may be polarized atany angle with respect to the other. This is accomplished by rotating ahalf wave plate 1705 (which is internal to the beam splitter 1703) to anarbitrary angle so that the beam leaving 1707 will now be polarized atan arbitrary angle with respect to beam 1706. After the beams leave thebeam splitter 1703 they strike diffusers 1708 and subsequently aredetected by photodetectors 1709 and 1710. The advantage of this type ofbeam splitter 1703 is that the outgoing beams may be polarized in thesame direction. As a result when the beams 1706 and 1707 strike thediffusers 1708 and photodetectors 1709 and 1710 the reflection fromthese surfaces will be identical and the detected signals will haveidentical reduction due to surface reflection. This fact makes thecalibration of the instrument considerably easier. The computation ofthe phase shift of the incoming beam 1701 is proportional to thedifference in the amplitude of the two beams as measured by thephotodetectors 1709 and 1710.

The incoming laser beams discussed in previous paragraphs have beendescribed as P, S or 45° polarized beams. These earlier discussions arepreferred embodiments of this invention. It is also possible toilluminate the surface with unpolarized light and detect the resultingreflected signals with the same optical and electronic methods. Theresulting detected signals, which use a source of light which isunpolarized, will still give measurements of the phase shift,topography, reflectivity, defects and particles.

A problem with conventional optical profilometers is that they arematerial (reflectivity) dependent. That is, a step height includingchrome on a glass substrate will give a different optically measuredheight than the same step height of chrome on a chrome substrate. Oneembodiment of the present invention removes this limitation. A reasonthat the optical profilometer shown in FIGS. 3-6 is material dependentis shown in FIG. 18. FIG. 18 shows the reflectivity versus angle forcopper and glass for S, P and randomly polarized light. When a laserbeam is focused onto a glass sample the beam will include a range ofangles whose magnitude will depend upon the numerical aperture of thefocusing lens. If a modest numerical aperture of 0.13 is assumed thenthe range of angles will be 15°. If the angle of incidence is 58° thenthe angles incident will vary from 51° to 66°. As a result, thereflected beam will have an intensity variation across its profile givenby the reflection coefficient of the material versus angle between 51and 66° multiplied by the incident intensity variation. For example, inthe case of glass the S reflection coefficient varies from 11 to 23%over this range of angles. Copper on the other hand will have an Sreflection coefficient variation from 82% to 88% over the same anglerange. The net result of this is that the centroid of the beam will beshifted towards larger angles for both copper and glass, but the shiftis much greater for glass than for copper. As a result when the focusedbeam is scanned from glass to copper the centroid of the beam on thequadrant detector will shift showing an apparent height change when infact there is no change in height.

One way to reduce (but not eliminate) the material (reflectivity) effectis to use randomly polarized light. For this discussion, randomlypolarized light is equivalent to circularly or 45° linearly polarizedlight. FIG. 18 shows that randomly (or circularly or 45° linear)polarized light has much less variation with incident angle when theangle of incidence is less than 45°. As a result the effect of material(reflectivity) may be reduced in designs which would otherwise show astrong reflectivity dependency by using an angle of incidence (θ) whichis less than 45° together with light which is randomly, circularly or45° linearly polarized. If the above criteria are applied to the designsshown in FIGS. 3-6, 27, 28, 29 and 31 then the material (reflectivity)dependency will be reduced.

The embodiments shown in FIGS. 19 through 26 completely remove thematerial (reflectivity) dependency by the use of a retro-reflector. Theembodiments shown in FIGS. 19 and 20 include an S polarized laser diode1901 which is split by a 50/50 non-polarizing beam splitter 1903 anddirected onto a focusing lens 1904 which focuses the beam 1906 to asmall spot on a substrate 1907 which may be a silicon wafer, thin filmdisk or optical substrate. The beam reflects from the substrate and isrecollimated by a second lens 1904 and then reflects from aretro-reflector 1905. The retro-reflector 1905 may be a conventionalretro-reflecting prism (a Porro prism) or a conventional cube cornerprism which are both available from CVI, Inc. Albuquerque, N. Mex.

The retro-reflected beam is then refocused upon the substrate andreflects a second time and then passes to the quadrant detector 1902where the height and slope are measured. The double reflection from thesubstrate removes the material dependency from the optical signal. Theretro-reflector takes the first reflection from the surface and invertsthe beam profile and reflects it back to the surface where it undergoesa second reflection. The second reflection alters the beam profile inexactly the opposite manner of the first reflection, since the beamprofile has been inverted by the retro-reflector. As a result the doublyreflected beam has a symmetric profile and its centroid is not shiftedregardless of the material type, reflectivity, polarization, angle ofincidence or range of angles in the beam. However, if there is a heightchange present then the amount of height change will be doubled by theretro-reflector.

A change in the beam profile with reflection from a surface is shown inFIGS. 34A-34C. In FIG. 34A a uniform beam profile has been chosen forillustrative purposes. In an actual device the beam profile would have agaussian shape. After one reflection from the surface underinvestigation the portion of the beam coming from larger angles ofincidence (on the right in FIG. 34B) will have a greater intensity asshown by the profile illustrated in FIG. 34B. It is the non-uniformintensity shown in FIG. 34B which results in a centroid shift of thebeam even in the absence of any height change. When the beam strikes theretro-reflector the profile is inverted and redirected towards thesurface where it undergoes a second reflection. The second reflectionproduces the symmetric beam profile shown in FIG. 34C. The symmetricbeam profile produced by the two reflections and the retro-reflectordoes not have a centroid shift when there is no height change regardlessof the material from which the beam is reflecting.

Using a second optical head that is mirror imaged about the focal point,as shown in FIG. 20, allows the separation of the slope and height. InFIG. 20, 2001 is the S polarized laser diode, 2002 the quadrantdetector, 2003 the 50/50 non-polarizing beam splitter, 2004 the focusinglenses, 2005 the retro-reflector and 2006 the focused beam. When theseoptical heads are combined as shown in FIG. 22 and the outputs areadded, the slope signals will cancel and the height signals will add. InFIG. 22 2201 are the S polarized lasers, 2202 the quadrant detectors,2203 the 50/50 non-polarizing beam splitters, 2204 the focusing andcollimating lenses, 2205 the retro-reflectors. The separation angle φ isgenerally set to be less than 10°. The quadrant detectors may bereplaced with bi-cell detectors with the split-oriented perpendicular tothe plane of incidence.

The sensitivity is increased by using a higher angle of incidence, theretro-reflector and adding the outputs of the mirror imaged headstogether. Theoretically the sensitivity can be increased to 8 times theactual surface height. This would require an incidence angle of 900, inpractice one can get a sensitivity increase of 6.9 by using an incidenceangle of 60° with retro-reflectors and summing two mirror images heads.This results in an optical profilometer that can achieve high lateralresolution, high sensitivity, measure 90° step heights, is materialindependent and separates the slope and height signals.

An alternate embodiment to the design shown in FIG. 19 is given by FIG.21. This design uses an S polarized laser 2101 that is directed onto apolarizing beam splitter 2102. The S polarized beam is completelyreflected by the polarizing beam splitter and passes through a quarterwave plate 2103 which is oriented such that circularly polarized lightis focused onto the substrate. The circularly polarized light isretro-reflected and passes through the quarter wave plate 2103 a secondtime at which point it becomes P polarized and passes through thepolarizing beam splitter without reflection and impinges upon thequadrant detector. This design is much more optically efficient thanthat shown in FIG. 19 and no beam reflects back towards the laser 2101.The disadvantage of this design is that the signal reflected from thesurface for circularly polarized light is less than for S polarizedlight. The design shown in FIG. 21 may be used to replace the elementsin FIG. 22 so that an optical profilometer that uses circularlypolarized light is created.

Another embodiment of a material independent optical profilometer isshown in FIG. 23. This embodiment uses a single S polarized laser diode2301 and a 50/50 non-polarizing beam splitter 2302. The split beams aredirected onto a pair of 50/50 non-polarizing beam splitters 2303 andthen focused upon the substrate. The advantage of this design is that ituses a single laser diode. This design may also use the circularlypolarized elements shown in FIG. 21.

FIG. 24 shows another embodiment of a material independent opticalprofilometer. This design is similar to FIG. 22 but in this case thebeams do not overlap. The individual elements 2401 and 2402 aremechanically attached together and scanned over a substrate at the sametime. The resulting two images are aligned with software in order toaccount for the difference in the position of the two beams on thesubstrate. Once the beams have been aligned in software the images areadded so that slope and height may be separated as discussed in earlierparagraphs. This design may also use the circularly polarized elementsshown in FIG. 21.

FIG. 25 shows another embodiment of an optical profilometer thatseparates slope and height and is material independent. This design usesonly two lenses instead of the four used in FIGS. 22, 23 and 24. Theadvantage of this design is that it uses fewer optical components. Thisdesign may also use the circularly polarized elements shown in FIG. 21.

FIG. 26 shows a profilometer design that is used to eliminate theeffects of the semiconductor pattern. When a patterned semiconductorwafer is rotated beneath a fixed beam whose plane of incidence isoriented in the radial or circumferential direction as shown in FIG. 4,artifacts will be created in the data which are dependent upon theorientation of the semiconductor pattern. These effects may beeliminated by the design shown in FIG. 26 that includes acircumferentially oriented profiler 2601 and a radially orientedprofiler 2602. If both these heads are used to simultaneously scan thesurface of the patterned wafer and then the separate outputs from 2601and 2602 are added together then the resulting data will be independentof the pattern on the semiconductor wafer. This design may also use thecircularly polarized elements shown in FIG. 21.

Another problem with conventional optical profilometers is that they maygive incorrect results when attempting to measure steps or profiles onthin transparent layers. This is because the bottom surface reflectionfrom the thin transparent layer gives a spurious signal that is added tothe signal from the top surface. This problem can be solved by using adeep UV wavelength (for example 266 nm) for the laser in FIG. 19 (1901)where nearly all transparent materials are strongly absorbing. If a deepUV laser is used then there will be no bottom surface reflection sincethe thin transparent layer will absorb the UV signal. An additionaladvantage of using a 266-nm laser is that it can be focused to a beamsize of approximately 0.2 microns resulting in a lateral resolution of2000 Å.

FIG. 27 shows an optical profiler design that also separates slope andheight. This design will be sensitive to material (reflectivity) changesbut these effects can be minimized by choosing an incidence angle lessthan 45°, operating with a modest numerical aperture and using random,circular or 45° linear polarization, as discussed earlier. Thisembodiment begins with a random, circular or 45° linearly polarized (asshown) laser 2701 that is incident upon a polarizing beam splitter 2708.The P component is transmitted and is rotated to S polarization by thehalf wave plate 2707. This counter clockwise propagating beam continuesto the right and totally reflects from polarizing beam splitter 2708 andpasses through a quarter wave plate 2709 which is oriented to producecircularly polarized light which is directed onto the substrate 2711 bya turning mirror 2706. A lens 2705 focuses the beam and after reflectingit is recollimated by a second identical lens 2705. The beam then isdirected onto a second turning mirror 2706 and passes through a secondquarter wave plate 2704 which is oriented to produce P polarization. TheP polarized beam passes through the polarizing beam splitter 2708 andimpinges upon the quadrant detector 2702. A similar path is followed bythe clockwise propagating beam with this beam impinging upon the rightquadrant detector 2703.

When the substrate 2711 changes slope as indicated by 2710 the clockwise(CW) and counter clockwise (CCW) beams will move in the same directionon the detectors 2702 and 2703. For the slope change shown with 2710both CW and CCW beams will move to the right on detectors 2702 and 2703.When there is a height change then the CW and CCW beams will move inopposite directions on the detectors 2702 and 2703. For example, if thesubstrate plane 2711 moves up then the CCW beam on 2702 will move to theright and the CW beam will move to the left on 2703. As a result, if theoutputs of 2702 and 2703 are subtracted the slope signals will canceland the height signals will add. This design will be insensitive toslope changes and will have double the height sensitivity.

FIG. 28 shows another embodiment of an optical profilometer thatseparates slope and height. This design will be sensitive to material(reflectivity) changes but these effects can be minimized by choosing anincidence angle less than 45°, operating with a modest numericalaperture and using random, circular or 45° linear polarization, asdiscuss earlier. This design begins with a 45° linearly polarized laser2801 that is directed onto a 50/50 non-polarizing beam splitter 2802.The reflected beam is directed onto a polarizing beam splitter 2803. Thepolarizing beam splitter 2803 may be a Glan-Thompson or a polarizingcube beam splitter or any similar polarizing beam splitter. The splitbeam is separated into a P polarized component which propagates counterclockwise (CCW) and an S polarized component which propagates clockwise(CW). The P polarized CCW beam is rotated by a half wave plate 2804 sothat it becomes S polarized and then any remaining non S polarizedintensity is removed by an S oriented polarizer 2805. The CCW beam isdirected onto a focusing lens 2807 by a turning mirror 2806 and isreflected from the substrate 2808. The CCW beam is recollimated byanother identical lens 2809 and reflects from another turning mirror2810 and passes through another S polarizer 2811 and then reflects fromthe polarizing beam splitter 2803. The resulting beam is directed to thebeam splitter 2802 and a portion passes through and impinges upon thequadrant detector 2812. The quadrant detector may be replaced with abi-cell detector with the split perpendicular to the plane of incidence.

The CW propagating beam follows a path similar to the CCW beam afterreflecting from the beam splitter 2803. After the CW beam reflects fromthe substrate 2808 and passes through the polarizing beam splitter 2803a portion then passes through the non-polarizing beam splitter 2802 andimpinges upon the quadrant detector 2812. When the substrate has a slopethe CW and CCW beams will move apart (opposite directions) on thedetector 2812 and the output will be zero. When the substrate has aheight change the CW and CCW beams will move in the same direction onthe detector 2812 and the output will be double that of a single beam.The advantage of this design is that it uses a single detector,separates slope and height and gives twice the height signal. There willbe no interference of the CW and CCW beams on detector 2812 since theyare orthogonally polarized.

FIG. 29 shows another embodiment of an optical profilometer thatseparates slope and height. This design uses 45° linearly polarizedlight incident upon the substrate so as to minimize the effects ofmaterial differences. The design is similar to that shown in FIG. 28except the CCW beam encounters a half wave plate 2901 which is orientedto rotate the incident P light by 45° and this light passes through a45° oriented linear polarizer 2902. Upon reflecting from the substratethe beam passes through a second 45° oriented linear polarizer 2903 andthrough a half wave plate 2904 which is oriented to rotate thepolarization an additional 45° so that it becomes S polarized. The Spolarized light reflects completely from the polarizing beam splitterand is directed onto the quadrant detector 2905. If the angle ofincidence is set to approximately 30° and the numerical aperture chosento be about 0.13 then this design will reduce the effects of materialdifferences upon the height signal. The advantages of this design areits double height sensitivity, reduced material sensitivity, separationof height and slope, and single detector.

It is interesting to compare the advantages and disadvantages of thedesigns shown in FIG. 29 and FIG. 22. FIG. 29 is simpler since it uses asingle detector, but it has some material sensitivity. The design ofFIG. 29 is limited to relatively small numerical apertures because ofmaterial sensitivity whereas that of FIG. 22 may use any numericalaperture and still be material independent. The angle of incidence ofthe design of FIG. 29 is limited to less than 45° because of materialsensitivity. This reduces the sensitivity according to FIG. 7, whereasthe design of FIG. 22 does not suffer this sensitivity loss. Theretro-reflector on FIG. 22 and the double head design gives a four-foldincrease in the height sensitivity. This fact and angle of incidencemultiplier mean that the theoretical height sensitivity of the design ofFIG. 22 is 8 times the physical height change. The same analysis appliedto the design of FIG. 29 gives double the physical height change. Insummary, the design of FIG. 22 will be material insensitive, achievesperfect slope cancellation, may be run at much higher lateralresolution, and is 4 times more sensitive than that of FIG. 29. Theadvantage of the design of FIG. 29 is the simplicity of a singledetector. The designs of FIGS. 28 and 29 also achieve perfect slopecancellation since the path length for the CW and CCW beams areidentical. There is no interference of the CW and CCW beams on thedetector 2905 since the beams are orthogonally polarized.

FIG. 30 shows an optical design that measures only slope and rejectsheight and material (reflectivity) changes. This design is very similarto that shown in FIG. 28 except in this case a Wollaston prism 3001 actsas a polarizing beam splitter. The orientation of the prism 3001 meansthat both the CW and CCW beams strike the prism on the same side of thesplit in the prism. This fact means that any height (or material)changes will move in opposite directions on the detector and slopechanges will move in the same direction. As a result this design rejectsmaterial sensitivity and height changes and doubles the sensitivity toslope. This design is intended as a high sensitivity slope measurementdevice. The slope may be integrated to arrive at the topographicprofile.

The design shown in FIG. 31 is another embodiment of an optical profilerthat measures height and rejects slope. This design will be material(reflectivity) sensitivity. The design as shown uses S polarized lightbut it is also possible to use random, circular or 45° linearlypolarized light, for example. If one of these polarization's are usedand the angle of incidence θ is less than 45° then the materialsensitivity will be reduced. The design begins with a 45° linearlypolarized laser 3101 which is directed onto a 50/50 non-polarizing beamsplitter 3111 which directs a portion of the beam to a polarizing beamsplitter 3106. The splitter 3106 reflects the S portion of the beam anddirects it in the clockwise direction where it passes through a Spolarizer 3110 (to improve the linear polarization) and is deflectedonto a substrate 3107 by a turning mirror 3109. The CW beam is focusedby a lens 3104 and recollimated by an identical lens 3104 afterreflecting from the substrate 3107. The CW beam is deflected by aturning mirror 3103 and passes through another S polarizer 3102 andimpinges upon the polarizing beam splitter 3106 and is reflecteddownward onto a quarter waveplate/mirror combination 3105 which convertsthe S polarization to P which reflects from 3105 and passes through thepolarizing beam splitter 3106 and a portion passes through 3111 andimpinges upon the quadrant detector 3112. The CCW propagating beamfollows a similar path before impinging upon the quadrant detector 3112.A bi-cell detector may be substituted for the quadrant detector 3112with the split-oriented perpendicular to the plane of incidence.

The embodiment shown in FIG. 31 behaves in a manner similar to thoseembodiments shown in FIGS. 28 and 29. When the CW and CCW beam encountera slope 3108 in the substrate the beams move apart on the detector 3112.When a height change is encountered then the CW and CCW beams move inthe same direction on the detector 3112. As a result slope changes arecancelled and height changes are doubled. The advantage of this designis that it uses a single detector. This embodiment achieves nearlycomplete slope cancellation since the CW and CCW beam paths differ onlyby the length of the polarizing beam splitter 3106. There is nointerference of the CW and CCW beams on the detector 3112 since they areorthogonally polarized.

FIG. 32 shows an optically scanned embodiment of a material independentoptical profilometer. This embodiment uses a rotating polygon 3202(available from Lincoln Laser, Phoenix, Ariz.) or XY galvanometricscanner (available from GSI Lumonics, Watertown, Mass.), or anacousto-optic scanner (available from Electro-Optical Products Corp.,Fresh Meadows, N.Y.) to scan the beam from point A to B (the Xdirection). A pair of XY galvanometric scanners or acousto-opticscanners may be used to scan the beam in two dimensions. An alternativeis to use the design of FIG. 32 to scan in the X direction (from A to B)and a mechanical stage to scan in the Y direction (in and out of thepage). This design uses an S polarized laser diode 3201 that is directedonto a scanner 3202 that scans the beam 3209 in a clockwise motion. Thescanner is placed at the back focal plane of the scan lens 3206. Thatis, the scanner (in this case a rotating polygon) 3202 is placed onefocal length along the beam path from the scan lens 3206. The scannedbeam is incident upon a polarizing beam splitter 3204 and totallyreflects onto quarter wave plate 3205 and then a scan lens 3206 where itis scanned from points A to B. Upon reflecting from the substrate 3208at points A or B the beams pass through a second scan lens 3206 and areincident upon a retro-reflector 3207 placed at the back focal plane ofthe scan lens 3206. The retro-reflector causes the beams to retracetheir path and upon passing a second time through the quarter wave plate3205 the beam becomes P polarized and passes through the polarizing beamsplitter 3204 and is incident upon a quadrant detector 3203 which isplaced at a point near the back focal plane of the scan lens 3206. Thebeam is scanned across the quadrant detector 3203 in the directionshown.

In order to separate slope and height terms according to one embodimentof the present invention a mirror image the design is used at the top ofFIG. 32 and this is shown at the bottom of FIG. 32 as 3210. Thisembodiment 3210 can also have its polygon scanner rotating in aclockwise direction so that both beams will scan from points A to B.When the outputs from the quadrant detectors of the designs at the topand bottom of FIG. 32 are added together then the resulting signal willgive a signal which is proportional to the height of the substrate 3208and independent of its slope. That is, the slope terms will cancel andthe height terms will add. The signal will also be independent of thereflectivity of the material for the reasons discussed in earlierparagraphs. One aspect of the design of FIG. 32 is seen in the largedifference in path lengths of the beams that are directed at points Aand B. This means that in order for the beam to remain in focus overthis large path length difference this design is best utilized at lowresolution, since a large depth of focus (large spot size) design mustbe used.

An alternate embodiment that increases the resolution is shown in FIG.33 as 3301. In this design the laser diode 3201, polygon scanner 3202,and polarizing beam splitter 3204 of FIG. 32 are rotated by 90° so thatthe laser and polygon are now arranged as shown in FIG. 33, 3301. Allthe other components remain unchanged. The view of FIG. 33 is from thetop of device whereas that of FIG. 32 is from the side. The embodimentof 3301 scans the beam from points A to B and since the path lengthsfrom the scan lens to A and B are equal then a much higher (smaller spotsize) resolution optical design may be used. In order to separate slopeand height according to one embodiment of the present invention, amirror image 3301 about the line AB is used to create a second head(analogous to FIG. 32) as shown as 3302 whose quadrant detector outputis added to the quadrant detector of 3301. This will separate slope andheight in the same manner as FIG. 32. As shown in FIG. 33 the polygonscanner moves the beam in the Y direction. A mechanical stage (notshown) or a second polygon scanner may accomplish the X direction scan.

FIG. 36 shows a side view of another embodiment of a materialindependent optical profilometer that uses a single detector. Thisdesign is similar to that shown in FIGS. 19, 20, 21, 22, 23, 24, and 25except that only a single detector is required in FIG. 36. 3601 is aconventional polarizing beam splitter, 3602 is a linear polarizeroriented in the P orientation, 3603 is a linear polarizer oriented inthe S orientation, 3604 is a 90° polarization rotator, which may be ahalf wave plate or optically active quartz, 3605 and 3606 are turningmirrors, 3607 is a quadrant detector, 3608 is the beam from one half ofthe mirror imaged optical system and 3609 is the beam from the otherhalf of the optical system. The two halves of the mirror imaged opticalsystem are shown as the solid and dashed lines in FIG. 36. The solid anddashed halves of the optical system are not in the same plane as shownin FIG. 37. The advantage of the design of FIGS. 36 and 37 is that ituses only a single detector which eliminates the issue of achievingidentical (or substantially identical) detectors as used in the designsof FIGS. 19 through 25. FIG. 37 shows the design of FIG. 36 as seen fromthe top with the components labeled as in FIG. 36. The dashed beam inFIG. 37 corresponds to the dashed beam in FIG. 36. Note that the beamsthat strike the detector 3607 are orthogonally polarized and as a resultwill not interfere. The signal from the two beams 3608 and 3609 areoptically added when they strike the detector 3607. As a result thesignal output from 3607 will be proportional to the height of the objectimaged and independent of its reflectivity or slope.

The quadrant detectors shown in FIG. 3-6, 8-10, 12, 15, 19-33, 36 and 37may be replaced by conventional position sensitive detectors such asmodel S5991 available from Hamamatsu Photonics K.K., Hamamatsu City,Japan or by bi-cell detectors also available from Hamamatsu.

The detection of the optical signal is done over a bandwidth from DC to3 MHz since this is the bandwidth of the quadrant detectors as describedin the preceding text. This bandwidth may be filtered as appropriate toremove mechanical vibration, optical noise, or stray light signals. Analternate detection scheme is to modulate the laser intensity and tosynchronously detect the signal from the quadrant detectors at the lasermodulation frequency. This method will greatly improve the signal tonoise of the detected signal and will reject external noise sources suchas vibration. The disadvantage of this approach is that the speed ofdata acquisition will be greatly reduced.

A multiple spot size optical surface analyzer is shown in FIG. 38according to one embodiment of the present invention. The opticalsurface analyzer includes a laser diode with internal feedbackphotodiode 3801, a linear polarizer 3802, a half wave plate 3803 with amotor 3815 for rotating the half wave plate so that P, S and 45°polarization is available. The optical surface analyzer also includes aGalilean or Keplerian telescope 3816 for expanding or diminishing thebeam diameter. This telescope 3816 may be moved in and out of the beamvia a motor 3817.

Multiple beam diameters may be achieved by moving differentmagnification Galilean or Keplerian telescopes into the beam via themotor 3817. The embodiment of FIG. 38 shows only two possible beamdiameters (the original beam diameter and the expanded one with 3816present in the beam), however any number of beam diameters may beachieved by using a series of different magnification Galilean orKeplerian telescopes attached to the motor 3817. An alternativeembodiment can use a continuously variable magnification telescope suchas model K61-386 available from Edmund Industrial Optics, Barrington,N.J., USA. If this type of telescope is used to replace 3816 then acontinuous range of beam diameters and hence focussed spot sizes ispossible. The motor 3817 can be computer controlled to adjust thecontinuously variable magnification telescope 3816 to give the desiredbeam diameter and hence spot size.

The different beam diameter will change the focussed spot size on thesubstrate 3806 in direct proportion to the magnification or diminutionproduced by the telescope 3816. With reference to FIG. 38, the systemincludes a focussing lens 3804, a turning mirror 3805, the substrate3806, a spindle motor 3807, a turning mirror 3808, a collimating lens3809, a quarter wave plate 3810, a quadrant detector 3811, a polarizingbeam splitter 3812 rotated at 45° to the plane of incidence, a quadrantdetector 3813, and a scattered light detector 3814 which may be a PMTtube, a PIN photodiode or an avalanche photodiode.

Some of the advantages of this design are that multiple focussed spotsizes are available in a single optical system and the system does notneed to be refocussed when a different beam diameter is selected. Thisis because the beam diameter is selected before the beam is focussed andthe incoming beam is always collimated regardless of magnification. Theadvantage of multiple spot sizes is that smaller spot sizes generallygive better sensitivity and resolution but slower throughput. A systemwhich has multiple spot sizes can be automatically configured to thedesired sensitivity and thoughput by a simple choice of spot size. Themotor 3817 is controlled by a connection to a small computer so that thebeam diameter (and hence spot size) may be selected by commands given tothe computer. The multiple spot size idea may be applied to the multiplebeam designs described in FIGS. 16, 19, 20, 21, 22, 23, 24, 25, 26, 27,28, 29, 30, 31, 32, 33, 34, 36, and 37.

Another problem in the inspection of disk drive media and wafers isdetermination of the thickness of different drive media and wafers. FIG.39 shows an embodiment of a system and method for measuring thethickness of thin film disks, wafers, substrate, or other substantiallyplanar objects. In this embodiment, laser diode 3901 transmits abeam3902 towards a bi-cell 3904 and 3905 or a quad-cell or a pair ofseparate photo diodes or another position sensitive detector. In anotherembodiment, the light beam source for beam 3902 may be a collimatedlight source. In the embodiment depicted in FIG. 39, beam 3902 isdirected substantially parallel to the surface of the disk or wafer andpositioned so that the disk or wafer 3907 is at the center of the beam3902. In other embodiments, other positions of beam 3902 relative to thedisk or wafer may be selected so long as portions of the beam pass bothabove and below the wafer or disk. The amount of light beam intensityreceived at top detector 3904 depends on the thickness of the disk. Asthe disk thickness increases, more of the laser beam is blocked by thedisk or wafer 3907 resulting in a decrease in the amount of laser beamintensity reaching top detector 3904. In response to the impinging lightbeam intensity, the detector generates a signal proportional to theamount of light beam intensity received at the top detector. Thicknessmask 3903 blocks unecessary light from the laser to aviod saturating thetop detector and to increase the sensitivity of the system to changes inthickness. The reference mask 3906 is positioned such that the bottomdetector 3905 receives the same amount of light beam intensityregardless of the disk thickness. The reading from the bottom detector3905 is used to normalize for any drift in the laser power, electronicsor receiving optics.

FIG. 40 shows the thickness detector integrated with an optical surfaceanalyzer 4005. The signal from the thickness detector is fed back vialines 4003 and 4004 to a processing device 4002. Any of a variety ofprocesing devices 4002 can be used, such as a microprocessor or apersonal computer. In one embodiment, the processing device 4002analyzes the data and determines the disk or wafer thickness from thedata. The processing device 4002 then commands a motor 4001 (via line4006) which is attached to the optical surface analyzer 4005. The motor4001 raises or lowers the optical surface analyzer 4005 in the Zdirection so as to compensate for any increase or decrease in the diskor wafer thickness as measured by the thickness detector. In this mannerthe optical surface analyzer 4005 will automatically remain in focusduring optical surface analysis of substrates with differentthicknesses. This invention also prevents the optical head 4005 frominadvertantly crashing into a thick disk or wafer since the distancefrom the optical head to the wafer is automatically maintained at afixed distance. The shadow technique used in this invention is effectivefor measuring the thickness of both opaque and transparent disks orwafers.

As the technology for the semiconductor and the disk drive industriescontinues to advance there is a need to detect and classify ever smallerdefects. Examples of such defects include scratches, pits and particles.When scratches have a length greater than the dimension of the laserspot projected on the surface of the object, it is possible to classifythe scratch based upon its detected aspect ratio. That is, the scratchis longer than it is wide. When a scratch is smaller than the dimensionof the laser spot this type of classification is not possible. As aresult, in conventional systems it is not possible to classify a smallscratch as different from a particle. The present invention presents atechnique to detect and classify scratches, pits, particles, and otherdefects which are smaller than the laser beam spot size.

The optical device shown in FIG. 38 can be arranged so that there aretwo identical sets of optics with orthogonal planes of incidence. In anembodiment, the plane of incidence of laser beam one is in thecircumferential direction and the plane of incidence of laser beam twois in the radial direction as shown in FIGS. 4 and 16. The scattereddetector 314 (on FIG. 3) and 3814 (on FIG. 38) will receive scatteredlight from both beams as the optical device is moved over the surface ofthe spinning wafer or disk. Scratches which are oriented perpendicularto the plane of incidence of the light will generate a strong scattersignal, while those oriented parallel to the plane of incidence willgenerate substantially no scattered signal. For example, acircumferentially oriented scratch will generate a strong scatter signalwhen the radial beam (laser 2 in FIG. 4) crosses it and substantially noscatter when the circumferential beam (laser 1 in FIG. 4) crosses it. Bycontrast, a particle which is substantially isotropic in shape willgenerate scatter from both the radial and circumferential laser beams.This is illustrated in FIG. 41 where a dual beam system consisting oftwo set of the optics shown in FIG. 38 are arranged with one laser beamin the radial plane of incidence and one in the circumferential plane ofincidence of the wafer or disk 4104 shown in FIG. 41. This laserorientation is also illustrated in FIGS. 4 and 16. The radial plane ofincidence beam is displaced from the circumferential plane of incidencebeam in such a way that the wafer or disk encounters it first. Theradial and circumferential plane of incidence beams are also displacedslightly in the circumferential (angular) direction as shown in FIG. 41.The distance between the beams is fixed and constant. Since a smallparticle is substantially isotropic it will scatter substantiallyequally when illuminated by the radial or circumferential beam as shownby the two black hexagons in 4101. The black hexagon of 4101 closest tothe OD of the wafer or disk is the signal received by detector 3814resulting from the radial beam illuminating the particle. The blackhexagon of 4101 nearer the ID of the wafer or disk is that resultingfrom the circumferential beam illuminating the particle. In the case ofa substantially isotropic particle the signals from both black hexagonsof 4101 are substantially equal. The case of a circumferentiallyoriented scratch is shown in 4103 where the radial beam gives a strongsignal and the circumferential beam gives substantially no signal. Thisis because the scratch is strongly anisotropic in its scatteringcharacteristics. That is, a beam whose plane of incidence is orientedperpendicular to the long direction of the scratch scatters much morethan a beam whose plane of incidence is parallel to the long directionof the scratch. The scattering from a scratch is also polarizationdependent. At an angle of incidence of approximately 60° a P linearpolarized radial beam will produce the largest scattered signal from acircumferentially oriented scratch. P linear is the preferredpolarization but S polarization, 45° linear polarization, circular orelliptical polarization will also give acceptable signals. In 4103 theradial beam gives a strong scattered signal from the scratch and if onelooks for a signal at the beam separation distance corresponding to thecircumferential beam one finds substantially no signal for thiscircumferential scratch. Here, the beam separation distance refers tothe time required for an object location to travel from the position ofthe first beam to the position of the second beam. The length of thisoffset time between the two signals will be dependent on the offsetdistance between the beams and the rotational speed of the wafer. Thecase for a small oval shaped pit whose major axis is along thecircumferential direction is shown in 4102. In this case the radial beamgive a strong signal as shown by the black hexagon in 4102 and thecircumferential beam give a small signal as shown by the gray hexagon.The ratio of the scattered amplitudes of the black and gray hexagonswill discriminate the pit from a scratch or particle. This isillustrated in FIG. 42 which shows at the top the relative amplitudes ofthe scattered light of the particle in the radial and circumferentialbeams. In this case the amplitudes are substantially equal. The middlepicture in FIG. 42 shows the amplitudes for a small oval shaped pit. Theamplitudes are not equal. The bottom picture in FIG. 42 shows theamplitudes for a circumferentially oriented scratch. In this case thescatter signal comes substantially only from the radial beam. If thescratch were oriented in the radial direction the scatter signal wouldcome substantially from the circumferential beam and substantially nonefrom the radial beam. If the scratch were oriented at 45° to the radialdirection then the scattered signal would be equal from both the radialand circumferential beams. It is also possible to orient the planes ofthe laser beams at angles other than 90°. In another embodiment, theorthogonal pair of beams may be oriented at an angle to the radial andcircumferential directions. In this manner one may more easily detectscratches which lie at directions which are neither radial norcircumferential.

Note that in the case where a scratch (which is smaller than the beamsize) or other defect is oriented at 45° to the radial direction, asystem involving two orthogonally oriented beams will not be able todistinguish a particle from a pit or scratch. More generally, when thelong axis of an anisotropic defect bisects the angle formed by theplanes of incidence between the two incident beams, the resultingscattered signals will not readily distinguish particles fromanisotropic defects such as scratches. In these situations, the truenature of the defect may be determined by generating a third scatteredsignal and a fourth scattered signal. In an embodiment, these scatteredsignals should be generated by incident beams that lie in planes whichare at 45° relative to the planes of the first two incident beams. Moregenerally, the third and fourth scattered signals should be generated byincident beams which lie in planes distinct form the planes of the firstand second incident beams. In an embodiment, the third and fourth beamsmay be generated by a third and fourth laser or other beam source. Inanother embodiment, the third and fourth beams may be generated bychanging the plane of incidence of both sets of optics. Another possibleembodiment is to continuously rotate the plane of incidence of theorthogonal beams while scanning the disk or wafer. In this manner allpossible planes of incidence may be incident upon the wafer or disk.Other possible embodiments will be apparent to those skilled in the art.

The scatter detector 3814 in FIG. 38 may have a condensor lens in frontof it to increase the cone of scatter angles that it may receive. Analternative embodiment is to place the scatter detector 3814 on top of ahole in an integrating sphere. The two laser beams pass through theintegrating sphere and all the scattered light angles are gathered bythe integrating sphere and measured by the detector 3814 at the top ofthe sphere. The detector 3814 may be a silicon or germanium photodiode,an avalanche photodiode or a photo multiplier tube.

The advantages of this technique are improved sensitivity to scratches,pits and particles, (which may be smaller than the beam spot size),improved ability to identify (classify) scratches, pits, and particles,and no blind spot to scratches, (as would be the case of acircumferential beam and a circumferential scratch).

While the invention has been particularly shown and described withreference to a preferred embodiment and various alternate embodiments,it will be understood by persons skilled in the relevant art thatvarious changes in form and details can be made therein withoutdeparting from the spirit and scope of the invention.

1. A method for measuring a defect on a surface of an object,comprising: directing a first light beam in a first plane of incidencetoward a first position on the object; directing a second light beam ina second plane of incidence toward a second position on the object,wherein the angle between the first plane of incidence and the secondplane of incidence is at an angle that is not one of substantially zerodegrees, subtantially 45 degrees and subtantially 135 degrees; detectinga first scattered light intensity, wherein the first scattered lightintensity comprises light intensity scattered from the first position;detecting a second scattered light intensity, wherein the secondscattered light intensity comprises light intensity scattered from thesecond position; comparing the first scattered light intensity and thesecond scattered light intensity to determine a first aspect ratio of adefect on the surface of the object.
 2. The method of claim 1, whereinthe first light beam and second light beam comprise polarized lightbeams.
 3. The method of claim 1, wherein the first light beam and secondlight beam comprise P-polarized light beams.
 4. The method of claim 1,wherein the angle between the first plane of incidence and the secondplane of incidence is 90 degrees.
 5. The method of claim 1, wherein thefirst plane of incidence is directed along the radial direction and thesecond plane of incidence is directed along the circumferentialdirection.
 6. The method of claim 1, wherein the first scattered lightintensity is detected a fixed offset time before the second scatteredlight intensity.
 7. The method of claim 6, wherein the fixed offset timeis determined based on a rotational speed of the measured object.
 8. Themethod of claim 6, wherein the first position is offset from the secondposition by a fixed offset distance.
 9. The method of claim 8, whereinthe fixed offset time is determined based on the fixed offset distance.10. The method of claim 1, wherein the first light beam is produced by alaser.
 11. The method of claim 1, wherein the second light beam isproduced by a laser.
 12. The method of claim 1, wherein the firstscattered light intensity is produced by scattering from a defect on thesurface of the object.
 13. The method of claim 12, wherein the firstlight beam strikes the object at the first position to make a first beamspot, and wherein the first beam spot is larger in size than the defect.14. The method of claim 1, wherein the first scattered light intensityand the second scattered light intensity are detected by a detector. 15.The method of claim 14, wherein the first scattered light intensitypasses through a condensing lens prior to detection by the detector. 16.The method of claim 14, wherein the first scattered light intensitypasses through an integrating sphere prior to detection by the detector.17. The method of claim 1, wherein the defect is a pit.
 18. The methodof claim 1, wherein the defect is a scratch.
 19. The method of claim 1,wherein the defect is a particle.
 20. The method of claim 1, furthercomprising: directing a third light beam in a third plane of incidencetoward a third position on the object; directing a fourth light beam ina fourth plane of incidence toward a fourth position on the object,wherein the angle between the planes of incidence of any two beams isnot equal to zero; detecting a third scattered light intensity, whereinthe third scattered light intensity comprises light intensity scatteredfrom the third position; detecting a fourth scattered light intensity,wherein the fourth scattered light intensity comprises light intensityscattered from the fourth position; comparing the third scattered lightintensity and the fourth scattered light intensity to determine a secondaspect ratio of a defect on the surface of the object.
 21. The method ofclaim 20, further comprising: calculating a combined aspect ratio of adefect on the surface of the object based on the first aspect ratio andthe second aspect ratio.
 22. The method of claim 20, wherein the anglebetween the third plane of incidence and the fourth plane of incidenceis 90 degrees.
 23. A method for measuring a defect on a surface of anobject, comprising: directing a first light beam in a first plane ofincidence toward a first position on the object; directing a secondlight beam in a second plane of incidence toward a second position onthe object, wherein the first plane position is offset from the secondposition by a fixed distance, and wherein the angle between the firstplane of incidence and the second plane of incidence is at an angle thatis not one of subtantially zero degrees, subtantially 45 degrees andsubtantially 135 degrees; detecting a first scattered light intensity,from the first position at a first detection time; detecting a secondscattered light intensity from the second position at a second detectiontime; comparing the first scattered light intensity and the secondscattered light intensity to determine a first aspect ratio of a defecton the surface of the object.
 24. The method of claim 23, wherein thefirst scattered light intensity is detected a fixed offset time beforethe second scrattered light intensity is detected.
 25. The method ofclaim 24, wherein the fixed offset time is determined based on saidfixed offset distance and a rotational speed of the measured object.